1. Field of the Invention
During the process of drilling a borehole in the search for producible hydrocarbons, the practice of drilling at an angle to the vertical (inclination) and in a specific direction (azimuth) is becoming more and more common. This so called directional drilling is usually performed in order to target a specific region in the earth (i.e., a specific geological formation) or to avoid hitting an object such as another well. In order to know where the borehole is being drilled so that its course may be corrected, downhole surveying instruments have been developed and employed. Such surveying instruments typically include a multi-axis accelerometer and a multi-axis magnetometer which measure at least two and preferably three components of the earth's gravitational and magnetic fields respectively. The azimuth and the inclination of the borehole at a particular point is then derived in a known manner from the information measured by the accelerometer and the magnetometer. In this process, it is assumed that the earth's gravitational and magnetic fields are unaffected by stray or spurious fields. If, however, the fields at the location of the measurement have been perturbed by extraneous fields, the determination of the azimuth and inclination of the borehole will be incorrect so that the driller will not know with the requisite degree of precision where the borehole actually is relative to his "target".
2. Description of the Prior Art
U.S. Pat. No. 4,163,324 describes a technique for correcting for errors that may affect a magnetic surveying instrument which result from stray magnetic fields arising from the equipment in the borehole. In that patent it is assumed that all interference is caused by magnetic material in the drillstring and is, therefore, axial. No means are provided for verifying the validity of this assumption. If the assumption is wrong, then the correction made to the azimuth measurement will also be wrong. Thus, the described technique would give incorrect results if the magnetic interference did not lie along the longitudinal axis of the tool as would be the case where the interference arose from an adjacent magnetic anomaly or from magnetized components in the tool having transverse magnetic fields.
U.S. Pat. No. 4,510,696 as well as the SPE/IADC publication 13476 "Reduction of Nonmagnetic Drill Collar Length Through Magnetic Azimuth Correction Technique" by A. W. Russel, point out that accelerometer and magnetometer measurements are subject to errors. This is especially true for the magnetometer whose measurement will be influenced by the presence of stray magnetic fields originating from magnetic materials in the vicinity of the magnetometer. The example cited by that patent is the magnetic field originating from the presence of the drill pipe which, unless made of non-magnetically permeable material, is likely to have a residual magnetic field. A further example is the stray magnetic fields that are present due to the proximity of other measurement equipment while drilling equipment containing magnetically permeable material.
While it is possible to place the surveying instrument within a non-magnetic drill collar with the instrument spaced sufficiently far from the origin of the stray field for the field to be small enough to have little or no influence on the measurements, such non-magnetic drill collars are costly. Indeed, it is the object of the above referenced patent and SPE/IADC paper to calculate the minimum length of non-magnetic drill collars (and thereby minimize the cost) that can be employed without incurring this interference.
Unfortunately, stray magnetic fields frequently result from components in the drilling assembly other than the drill collars. For example, various components of a downhole drilling motor or other downhole equipment may be magnetically permeable. Spacing these items sufficiently far from the surveying equipment may unacceptably constrain the design of the bottom hole assembly.
In an approach different from that of performing a calculation for determining the minimum size of non-magnetic drill collars, U.S. Pat. No. 4,682,421 describes a technique for detecting and removing constant magnetic biases from the transverse and axial magnetometer measurements. The technique requires the operator to make multiple surveys at various roll angles at a constant depth in a well in order to determine the cross axial component of the magnetic field bias due to magnetization of the drill collar.
In this technique, the X-axis magnetometer measurement is plotted versus the Y-axis magnetometer measurement (the Z-axis measurement lying along the longitudinal axis of the drill string) for each of the roll angles at which measurements are made. The resultant is a circle whose displacement from the origin is indicative of the horizontal X and Y magnetic biases. The longitudinal axis is then corrected by first subtracting the biases from the transverse axes and then computing the total magnetic field and the measured magnetic dip angle. The magnitude of the vector difference between the measured magnetic field and the tabulated magnetic field (obtained from a priori independent data) is then calculated. The vector difference between the measured magnetic field and the tabulated magnetic field is then used to obtain a corrected longitudinal magnetic field. Once these corrections are obtained they are applied to any individual survey to correct for the magnetic bias introduced into the measurement by the magnetic drill collar.
Several assumptions are implicit in the technique of U.S. Pat. No. 4,682,421. One assumption is that the transverse magnetic bias errors are fixed relative to the tool and are not due to, for instance, the position of some components of a mud motor which have additional internal degrees of freedom. A second assumption is that the magnetic bias errors are stable over time so that biases determined at the time of the calibration procedure can be used to correct data collected at other times. This assumption may not be entirely accurate in view of the shock and vibrations to which the drill string is subjected during the drilling process. A third assumption is that transverse bias errors dominate other causes of magnetic mismeasurement. For example, magnetometer alignment and scale factor errors are assumed to be small relative to the bias errors from stray fields when this may not be the case at all.
If these other error sources are the dominant cause of the resultant measurement error rather than the bias errors, the measured data will be degraded since erroneous biases will be subtracted from each sensor measurement. Likewise, if the magnetic biases are unstable, the data will also be corrupted.
U.S. Pat. No. 4,761,889 also describes a technique for addressing the problem of the effect of stray magnetic interference on a surveying device. This patent, as well as some of the other above mentioned techniques utilize a priori magnetic field magnitude and dip values to improve their results. None of the above mentioned techniques, however, attempts to take advantage of the additional a priori gravitational field strength information nor do they take into consideration the measurement uncertainties of the magnetometers and accelerometers.
The above techniques are therefore directed exclusively at magnetic field interferences and ignore potential errors in the accelerometer measurements. Since none of the above prior techniques takes into account measurement uncertainties that are design limitations of the magnetometer and accelerometer sensors themselves, they are unable to produce optimum results. Finally, none of the above techniques treats the three magnetometer axes and the three accelerometer axes on a equal basis. Rather, in those techniques, undue importance is given to the magnetometer z axis.
It would therefore be advantageous to have a technique for determining inclination and azimuth of a borehole that would beneficially utilize all of the a priori information available including gravitational field amplitude and measurement uncertainties. Desireably such a technique should perform corrections to the magnetometer and accelerometer measurements without requiring measurements from more than one orientation and one depth in the well bore in order to be self contained without having to assume that an interfering field is invariant from the beginning to the end of the survey of the borehole.
Furthermore it would be advantageous to have a technique which weighted the contributions of the measurements in accordance with the relative estimated uncertainties of the measurements but which otherwise treated all of the measurement axes on an equal basis. It would be of additional advantage to have a technique which could be varied in a manner such as to identify a faulty magnetometer or accelerometer axis. And finally, it would be of great advantage to have a technique that would be responsive not only to stray fields produced by objects within the borehole but which would have equal sensitivity to field anomalies originating outside of the borehole so that information relative to their location and distance could be obtained.